This research project will investigate risk factors for Kaposi's sarcoma (KS) associated with the acquired immune deficiency syndrome (AIDS). It is expected that the study population of incidents KS cases diagnosed at New York University Medical Center will consist primarily of homosexual or bisexual men; all KS cases diagnosed in the project's recruitment period, however, will be studied. Two main hypotheses will be investigated: First, that receptive anal-genital intercourse is the primary route of homosexual transmission of one or more KS/AIDS etiologic agents. Second, that one of more co-factors, such as exposure to unusual pathogenes or to heavy doses of an AIDS agent, predispose some persons with AIDS towards development of opportunistic infections (OIs). A case-control epidemiologic design will be employed to study the above issues. Cases will be approximately 150 persons diagnosed with KS/AIDS over a 12 month period. Two controls will be selected for each case, individually matched to the case on residence in the New York metropolitan area, age (+ 5 years), sex, sexual preference, race and primary physician. Cases and controls will be interviewed and blood will be drawn for analysis of IgG and IgM serum antibody titers go cytomegaloyirus (CMV), determination of presence or absence of HLA locus DR5, and for storage at -70 C. Univariate and multivariate logistic regression analyses will be conducted to investigate the roles of anal-genital intercourse compared with other forms of male homosexual sex in KS/AIDS transmission, and the roles of CMV, nitrites, HLA DR5 and other factors in the etiology of KS/AIDS. Follow-up of cases will be conducted for 1 year after diagnosis of disease to determine later development of OIs among persons originally diagnosed with KS; follow-up of controls will also be conducted for 1 year to determine the occurrence of AIDS in this group. Comparisons of persons diagnosed with KS only versus those with KS and OIs will be conducted to search for co-factors determining expression of OIs given AIDS. If anal-genital intercourse can be shown to be a primary route of transmission of KS/AIDS, prophylactic measures such as use of condoms or avoidance of anal-genital intercourse can be recommended with the possibility that some KS/AIDS cases may be prevented from occurring. If co-factors necessary for development of OIs given AIDS can be identified, then other measures to ameliorate effects of AIDS might become apparent.